ENERGY RELATIONSHIPS. 333 



olism. The increments are, however, sufficiently large to preclude any 

 attempt to measure the basal metabolism during the active stages of 

 digestion. Experimenters have therefore for years wisely insisted upon 

 the post-absorptive condition, that is, 12 hours after the last meal. 

 This is particularly necessary when the preceding diet has contained 

 liberal quantities of protein or an excessive amount of a mixed diet. 

 Our observations, in common with those of many others, show very 

 clearly that the effect of the ingestion of pure carbohydrate or of fat is 

 with normal individuals concluded in a relatively few hours ; were it not 

 for the protein in the diet, therefore, one might state that the post- 

 absorptive condition, or the so-called nuchtern condition, could be 

 obtained in a much shorter period than 12 hours. Insistence on the 

 12-hour period is, in all events, the wiser course. Even with this inter- 

 val, the injunction should be given to all subjects that excessive pro- 

 tein should not be taken in the last meal prior to the experimental 

 period. (See page 286.) 



RELATIONSHIP OF THE FUEL VALUE OF INGESTED FOOD TO EXCESS 



HEAT PRODUCTION. 



A relationship of unusual interest is that of the increase in the heat 

 production following the ingestion of food to that of the fuel value of the 

 food taken. While it may seem at first sight a gross misuse of engineer- 

 ing terms or terms of efficiency to apply them to the apportionment of 

 the caloric value of the ingested food of man, one might consider from 

 an engineering standpoint or from that of industrial efficiency that the 

 ingestion of food containing a certain number of calories would result 

 in a certain amount of excess heat. Excess heat production represents 

 an expenditure, either necessitated by the ingestion of food or resulting 

 from the ingestion of food, and hence may logically be attributed to and 

 in a sense chargeable to it. 



In considering the metabolism subsequent to the ingestion of food, 

 one should bear in mind the following facts: A considerable portion of 

 the diet, at least with ruminants, is distinctly indigestible, this portion 

 consisting of woody fiber, cellulose, etc. Secondly, only part of the 

 protein of the diet is oxidized inside the body. This is true of all ani- 

 mal life, the unoxidized portion of the protein molecule being with 

 mammals excreted chiefly in the form of urea. Furthermore, and this 

 applies more particularly to ruminants, fermentation processes take 

 place in the large intestine and cause a considerable production of 

 marsh gas and a liberation of heat as the result of bacterial action. 

 Finally, the ingestion of food per se causes an increase in the heat pro- 

 duction. It is clear, therefore, that a measure of the heat of combus- 

 tion of the intake has but little significance in relation to the ultimate 



